The present invention is related to the field of network management for data communications systems employing Dense Wavelength-Division Multiplexed (DWDM) communications links.
Dense Wavelength Division Multiplexing (DWDM) is a technology which uses densely packed wavelengths of light to effectively multiply the capacity of an optical fiber. The capacity of these systems is generally limited by fiber loss, unless optical amplifiers are used. A widely-deployed optical amplifier is the Erbium Doped Fiber Amplifier (EDFA), which amplifies light signals within an extremely large frequency band (xcx9c4 THz for a conventional EDFA at present). Although this frequency range is large, it is relatively small compared with the total low-loss window of present optical fibers. Thus, the capacity of optical-amplified systems is generally limited by EDFA bandwidth. A typical conventional band, or C-band, EDFA operates in the range of about 1530 nm to about 1560 nm. Newer L-band EDFAs operate in the range of about 1570 nm to about 1600 nm. Other amplifier technologies such as Raman and semi-conductor amplifiers may be used within the C and the L bands, as well as in other low loss regions of the fiber, e.g. xcx9c1260 nm to xcx9c1360nm, or in the xcx9c1400 to xcx9c1500 nm range (S-band) in newer fiber that lacks the well-known water absorption peak of traditional fiber.
A typical DWDM system employs a so-called Optical Supervisory Channel, or OSC, to provide communications among components in the DWDM system for purposes of operational control and monitoring. For example, it is necessary to monitor and control the output powers of the EDFAs in a system. An OSC typically takes the form of an optical signal of a given wavelength on each span of fiber in a link. At each amplifier node, the OSC is intercepted and converted to electrical form for local use. The electrical signals are also re-converted into optical form for transmission to the next amplifier node in the link.
It is widely known to use a so-called xe2x80x9cout-of-bandxe2x80x9d OSC in DWDM systems. According to this scheme, the OSC uses a wavelength that is widely separated from the wavelengths used to carry user data traffic. For example, in a C-band system in which the EDFAs operate over the range of wavelengths from 1530 to 1560 nm, the OSC may be placed at a wavelength of 1310 nm. This arrangement satisfies several needs in DWDM systems. The OSC does not consume any optical amplifier bandwidth or power which could be utilized by data-carrying optical signals. Also, an out-of-band OSC is fairly easy to extract from the composite DWDM signal. Extraction can be done inexpensively, with low loss, and with high efficiency. The latter is important to prevent mutual interference between the DWDM data signals and the OSC. Also, the wavelength tolerance for an out-of-band OSC can be fairly loose, so that less accurate, and therefore less expensive, lasers may be used. For instance, an out-of-band OSC might use an uncooled distributed feedback laser (DFB) whose wavelength might drift as much as +/xe2x88x92 5 nm over temperature and life. Finally, an out-of-band OSC continues to operate if the optical amplification fails or degrades.
Nevertheless, out-of-band OSC channels also have drawbacks that make them unattractive from the perspective of the needs of a user""s network management system. In many DWDM systems, the OSC is used for internal purposes and users are not given access to the OSC. Even when access is provided, there is no standardization of the communications protocols that are used for OSCs used by different vendors of DWDM systems. Any given OSC may use Ethernet, Internet Protocol (IP), HDLC, or other protocols, including proprietary protocols. The lack of standardization can create major complications when trying to interwork different OSCs and management channels. There can also be undesirable performance limitations if the amount of communications bandwidth required by a network management application exceeds the bandwidth provided by an OSC.
An example of a typical out-of-band OSC is shown in U.S. Pat. No. 5,914,794 of Fee et al., assigned at issuance to MCI Communications Corporation. In this patent, an OSC is transported on a 1510 nm wave, outside the traffic band of 1530-1560 nm. The OSC is terminated at every node, including each of several optical amplification stations. The terminated signal is converted to electrical form and processed by various elements, including a line supervisory module. Those messages intended for another node are included in an outgoing electrical signal which is converted to an optical signal and sent on the next optical xe2x80x9chopxe2x80x9d. The Fee patent suggests that the OSC can alternatively be carried in the traffic band, while presumably retaining the characteristic of being dropped, reconstituted, and added at every node in the optical network. Fee does not describe any problems or undesirable limitations that might exist in such an alternative implementation.
Aside from using an out-of-band OSC, there have been other approaches to providing a control channel in DWDM systems. One technique is referred to as an xe2x80x9cin-channelxe2x80x9d control channel, meaning that the control information is multiplexed in some manner with the data information. Variations of this technique include the use of overhead bytes in data frames; using a sub-carrier multiplexed tone on top of a data channel; and using code-division multiple access (CDMA) signals on top of a data channel. While these approaches can be useful in certain environments, they also suffer respective limitations. Overhead-based schemes do not work well in the context of xe2x80x9ctransparentxe2x80x9d services, which by definition provide t no user access to frame overhead bytes. Both sub-carrier multiplexed tones and CDMA signals complicate transceiver design, tend to degrade data channel performance, and offer only limited bandwidth. Another general approach is to use an out-of-fiber control channel, for example by using an entirely separate Internet Protocol (IP) network connecting user equipment together for network management purposes. Aside from the cost and complexity issues, this approach cannot be used to communicate with equipment located in remote areas, such as equipment located at an isolated regenerator site. Also, the lack of in-fiber signalling complicates the function of topology discovery, which is necessary for the proper functioning of network routing algorithms and other operations.
In accordance with the present invention, a wavelength-division multiplexed (WDM) system is disclosed that uses an in-fiber, in-band optical management channel (OMC). Many of the above-discussed drawbacks of other control channel architectures are avoided, so that users of the WDM system are able to create a comprehensive and robust network management system that can utilize substantial optical bandwidth to carry out its operations. Additionally, the OMC can be combined with a traditional OSC in a system in order to achieve additional benefits, including the ability to manage equipment in separate management xe2x80x9cdomainsxe2x80x9d that may exist in when the system includes equipment from different vendors.
The disclosed WDM system includes an optical communications link having cascaded optical amplifiers. Each optical amplifier has a pass band, throughout which the amplifier provides substantially a predetermined optical gain, so that each amplifier passes optical signals in the pass band along the optical communications link without interception. Transmitting optical communications equipment coupled to the input end of the link generates a WDM optical communications signal that includes a number of spaced-apart optical signals, including data optical signals and at least one management optical signal. The data optical signals and management optical signal occupy substantially the pass band of the optical amplifiers so as to be capable of passing through each amplifier to the output end of the link. Receiving optical communications equipment coupled to the output end of the optical communications link receives the WDM optical communications signal, separates the management optical signal from the data optical signals, recovers management information from the separated management optical signal, and uses the recovered management information to carry out management functions.
The in-band management optical signal passes through the optical amplifiers of the WDM system, and thus effectively carries end-to-end network management traffic. Moreover, the wavelength of the management optical signal is selected to minimize the impact on the user data signals. In a preferred embodiment, the management optical signal resides at an edge of the pass band of the optical amplifiers. For example, in a system having optical amplifiers specified to operate over the band from 1530 to 1560 nm, the management optical channel may be placed at 1529.5 or 1561 nm. A management optical channel placed in this manner generally receives less gain than the gain received by the data signals, and consequently may need to be operated at a lower data rate than the data channels. The higher-gain mid-band region is reserved for the data channels to maximize system data carrying capacity.
An in-fiber, in-band OMC can provide substantial bandwidth for use by a user""s network management system, without significantly degrading the performance of the data channels. Route discovery and other topology-dependent functions can be carried out in a straightforward manner, even when optical links traverse isolated areas where no separate communications networks are supported. Also, as mentioned above, the OMC can be used along with an OSC to achieve additional benefits, such as managing equipment in multiple management domains.
Other aspects, features, and advantages of the present invention are disclosed in the detailed description that follows.